For monitoring and maintenance of optical fiber lines of an optical fiber communication system, an optical pulse tester has been widely used. The optical pulse tester is an apparatus that can measure a loss distribution in a longitudinal direction of the optical fiber, and can detect a localized increase of loss or trouble of rupture of the optical fiber, as well. With the optical pulse tester, short optical pulses generated periodically are incident on the optical fiber, and the light intensity of a Rayleigh back-scattered light, which is produced by such short optical pulses on the optical fiber, is observed on a time base, thereby to measure a loss with respect to a distance. Details are described in a literature entitled "OPTICAL FIBER", Paragraph 12.4, published by Ohmsya, Ltd.
FIG. 9, FIG. 10 and FIG. 11 each shows a schematic structural block diagram of a conventional basic optical communication system, respectively. These conventional systems differ from one another in a manner of connecting the optical pulse tester with the optical fiber line. Station A serves as an optical sending terminal station comprising an optical sending apparatus 110, and station B serves as an optical receiving terminal station comprising an optical receiving apparatus 112, and stations A and B are respectively provided with optical pulse testers 114 and 116. Numeral 118 designates an optical fiber line.
In FIG. 9, an end 118a of the optical fiber line 118 on the side of the station A is adapted to be selectively connected with an output end of the optical sending apparatus 110 and an input/output end of the optical pulse tester 114, while an end 118b of the optical fiber line 118 on the side of the station B is adapted to be selectively connected with an input end of the optical receiving apparatus 112 and an input/output end of the optical pulse tester 116. Normally, the ends 118a and 118b of the optical fiber line 118 are respectively connected with the output end of the optical sending apparatus 110 and the input end of the optical receiving apparatus 112. Then when the necessity of performing tests arises, the end 18a of the optical fiber line 118 is connected with the input/output end of the pulse tester 114 or the end 118b of the optical fiber line 118 is connected with the input/output of the pulse tester 116.
Referring to FIG. 10, the optical pulse testers 114, 116 are always connected with the optical fiber line 118 by means of optical adding and dividing devices 120, 122. Thereby, this arrangement can be ready for use when conducting of tests becomes necessary, without switching over the connections as in the case of FIG. 9. Further, the output lights of the optical pulse testers 114, 116 may be made to have a waveband consisting of wavelengths different from a wavelength of the output light of the optical sending device 110, and this provides an advantage of measuring the optical fiber line 118 even during transmission of signals.
FIG. 11 shows an arrangement for making the best of the optical pulse testers 114, 116 in the case of having a plurality of optical fiber lines, wherein optical switches 124, 126 are provided to connect the output lights (and reflecting lights) of the optical pulse testers 114, 116 with a target optical fiber line (or the adding and dividing device connected thereto).
As described above, the optical tester is to observe the light intensity on a time base, so that it is necessary to convert the time base into a distance. A distance L with respect to a time t lapsed after the sending of pulses is expressed by: EQU L=ct/2n
wherein c is a light velocity and n is a coefficient which is called a group refractive index of optical fiber and referred to a propagation velocity of the optical signal when it advances through the optical fiber. The group refractive index n depends on design parameters and materials of the optical fiber. Generally, with the optical fiber having a zero dispersion wavelength in a 1.3 micron band, the n is in the order of 1.460.about.1.465, and with the optical fiber having a zero dispersion wavelength in a 1.55 micron band, the n is in the order of 1.470.about.1.475.
Accordingly, even if the pulse tester has a very high time accuracy, an error in the group refractive index n becomes an error in distance (uncertainty). The longer the distance of an observation point, the larger the absolute error becomes, and it is thought that with the above-mentioned parameters an error becomes as great as about 340 m for 100 km distance. The group refractive index n differs from one optical fiber to the other optical fiber, and it is possible to control each optical fiber constituting the optical fiber line, but this is not practical, because the data processing becomes quite complicated. Further, when a route for laying the optical fiber line is changed, comparison with previously observed data cannot be made, and, thus, an error in the actually laid position becomes greater.
Assuming, for example, the optical fiber cable is laid in a side-gutter along the rail way or the road, the gutter is covered by a lid after laying the optical fiber cable to protect the cable from an external influence. With this condition, if a trouble has occurred in a portion of the cable in this section, it is not possible to confirm the trouble by visual observation of a terrain appearance. In this case, for checking a possible fault location it is necessary to remove the lid of the gutter in a wide range including fore and after of the possible fault location, by taking into consideration a measuring error. Further, when the optical fiber cable is laid in and along the expressway, it is necessary to block or restrict traffic in a wide range of lanes to perform confirmation work of the possible fault location. Needless to say, such work becomes more difficult for embedded cables.
Further, in urban areas, the optical fiber cable is laid in and along a conduit under the road, and it is necessary to check the fault location by entering into a manhole provided in the public road. An interval between manholes is in the order of 100 m at the shortest, and considering the above-mentioned error of about 340 m in measuring the fault location, the manhole which is close to the fault location cannot be specified, and the fault location should be sought from, for example, manholes at four places. However, such work on the public road greatly affects the traffic network as it causes, for example, a long hours of traffic delay.
As such, with the precision of prior art, ascertaining the fault location requires many steps of operation, which is costly. Also, as a result, many hours are required until the communication is restored. Thus, in the case where the optical fiber cable is laid adjacent to or accompanying the public traffic network, the affect given to the traffic network is too great.